Effect of pyrolysis on Zhaotong lignite oxidation with aqueous sodium hypochlorite

Abstract The pyrolysis treatment of lignites could remove thermal-unstable aliphatic structures but enrich aromatic structures in resulting pyrolysis residues, which would facilitate producing benzene polycarboxylic acids (BPCAs). Herein, the demineralized Zhaotong lignite (DZL) was pyrolyzed at 250-550 °C to afford the corresponding pyrolysis residues. Subsequently, DZL and its pyrolysis residues were subjected to oxidation with aqueous sodium hypochlorite. By XRD, FTIR, and element analyses, it was found that the content and polycondensation degree of aromatic structures simultaneously increased with increasing pyrolysis temperature. Furthermore, the yield and selectivity of BPCAs both increased along with raising pyrolysis temperature in resulting products from fully oxidation of DZL and its pyrolysis residues. Based on the distribution of BPCAs, peri-condensed aromatic structures were dominant in DZL and its DPRs, and obviously increased with increasing pyrolysis temperature. More interestingly, good correlation was found between the yield of BPCAs and aromaticity parameters. Meanwhile, the yield of benzenepentacrboxylic acid was well associated with polycondensation degree parameters.